50 ' CHEEK TEETH 



the biting surface ends. The grinding teeth vary from simple 

 one - cusped teeth, precisely like canines, to teeth with an 

 enormous number of separate tubercles. In the former case it is 

 hard to distinguish between incisors, canines, and cheek teeth 

 in the lower jaw, where no suture separates the bone. More- 

 over it is quite common for the first cheek tooth in the lower 

 jaw to have the characters of a canine, while the true canine 

 approximates in its form to the antecedent incisors. This is 

 so, for instance, with the Lemurs, where the first premolar is 

 caniniform, and the canine shares in the curious procumbent 

 attitude which distinguishes the lower incisors of many of those 

 animals. 



A variable number of the anterior cheek teeth may be little 

 more than simple conical teeth ; but the rest of the set are 

 commonly more complicated. No definite laws can be laid down 

 as to the complication of the posterior as compared with the 

 anterior set. Broadly speaking, it is purely herbivorous creatures 

 in which the least difference can be detected at the two ex- 

 tremities, and which are at the same time the most elaborately 

 decorated with tubercles and ridges. The converse is true that 

 in purely carnivorous animals, including insect- and fish-eating 

 forms, there is the greatest difference between the anterior set 

 of grinding teeth and those which follow. In these two respects 

 such animals as a Lemur and a Rhinoceros occupy the extremes. 

 Furthermore, it may be said that omnivorous creatures lie, as 

 their diet would suggest, in an intermediate position. Generally 

 speaking, when there is a marked difference between the first 

 premolar and molars at the end of the series, there is a gradual 

 approximation in structure of a progressive kind. The tubercles 

 become more numerous in successive teeth ; but the corollary 

 which is apparently deducible from this, i.e. that the last molar 

 is the most elaborate of the series, is by no means always true. 

 The last cheek tooth indeed is often degenerate. On the other 

 hand, it is very markedly the largest of the series in such diverse 

 types as the Elephant, the hog Phacochoerus, and the Rodent 

 Hydrochoerus. It is a rule that the cheek teeth of the upper jaw 

 are more complicated than the corresponding teeth of the lower 

 jaw. 



The structure of the cheek teeth is very diverse among the 

 Mammalia. Broadly, two types are to be recognised. There are 



